The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance imaging apparatus that conducts parallel imaging.
In a magnetic resonance imaging (MRI) apparatus, a subject to be imaged is carried into an internal space of a magnet system, i.e., an imaging space in which a static magnetic field is generated, a gradient magnetic field and a radio frequency (RF) magnetic field are applied to excite spins within the subject to thereby generate magnetic resonance signals, and an image is reconstructed based on the received signals.
One scheme of the magnetic resonance imaging is parallel imaging. Parallel imaging is described by, for example, Klaas P. Pruessmann et al. in an article entitled “SENSE: Sensitivity Encoding for Fast MRI”, Magnetic Resonance in Medicine, 42: 952-962 (1999).
Generally, in parallel imaging, magnetic resonance signals are acquired via a plurality of receiver systems in a simultaneous and parallel manner. The acquisition of the magnetic resonance signals is conducted with a field-of-view (FOV) reduced by half, for example. By reducing the FOV by half, the rate of signal acquisition is doubled.
An image is reconstructed based on the signals thus acquired. The image reconstruction is conducted in two steps. At the first step, an intermediate image is produced based on the signals acquired by the plurality of receiver systems. The image production employs two-dimensional inverse Fourier transformation. The produced image has a reduced FOV. Because the FOV is reduced, aliasing images resulting from wraparound from outside of the FOV are contained in the image.
At the second step, the aliasing images are brought back to their original positions by applying a certain calculation to the image and an image with a whole FOV is produced. The calculation uses the following equation:V=(S*S)−1S*A,  [Equation 3]where                V: pixel values of the image with the whole FOV,        S: a sensitivity matrix,        S*: an adjoint matrix of S, and        A: pixel values of the intermediate image.        
The sensitivity matrix S is determined by the spatial distribution of the sensitivity of the plurality of receiver systems. The sensitivity of a receiver system generally has a complex form, and data of the sensitivity matrix therefore also has a complex form. Similarly, the pixel values V and A also have a complex form.
One technique for taking a cross-sectional image of the head is MS-DW-EPI (multi-shot diffusion-weighted echo planar imaging). The technique involves capturing a cross-sectional image to which a weight is applied such that spins with less diffusion have a larger signal intensity, by a multi-shot (MS) echo planar imaging (EPI) technique. The captured cross-sectional image is suitable for diagnosing the presence of cerebral infarction.
In the multi-shot echo planar imaging technique, the collection of imaging echoes filling one screen image is spread over a plurality of times. Specifically, the spins are excited a plurality of number of times, and a certain number of imaging echoes are collected during each excitation.
If positional shifting of the spins occurs between the excitations due to pulsation of the brain in such a process, a phase error between the imaging echoes leads to ghosts generated in the reconstructed image. To avoid this, a navigator echo is collected for each excitation, and the phase of the navigator echo is used to correct the phase of the imaging echoes.
Parallel imaging according to he MS-DW-EPI technique has not been conducted. This is because when the imaging echoes are phase-corrected by the navigator echo, the phase originating from the sensitivity of the receiver systems is unnecessarily corrected, resulting in inconsistency with the sensitivity matrix.